The invention relates to the processing of compositions containing magnetic particles, in which the particles are collected from a vessel containing the composition or are released as a composition in the vessel, and in which a magnetic rod is used to collect the particles to its tip or to release them from its tip. The invention relates specifically to the vessel and the rod used in the processing. The invention can be used for instance in various manufacturing, purification or analysis methods.
Magnetic particles are used in various chemical processes as a solid phase, to whose surface a given component adheres. Owing to the particles, the available surface of the solid phase becomes as large as possible. The particle size is typically in the range from 0.05 to 10 xcexcm. The particles can be moved by means of a magnetic field. Thus, for instance in a solution they may be transferred to the wall of a vessel, so that the remaining solution can be removed from the vessel by decanting or pipetting. The particles can also be separated from the solution by immersing a magnetic rod in the solution. Magnetic rods are also known, which comprise a vertically moving magnet within a cover. With the magnet in lower position, the particles can be collected on the rod surface, especially at its end. When the magnet is raised into upper position, the particles can be released from the rod accordingly.
WO 94/18565 discloses rods used for collecting and transferring magnetic particles. They are preferably equipped with a cover with a sharp tip enclosing a movable magnet. The vessel used jointly with the rod may have a bottom matching the shape of the rod tip. The interstice between the rod and the vessel wall is made narrow on purpose, in order to make the liquid flow as rapidly as possible between the rod and the vessel wall when the rod is moved vertically. This enhances mixing and mass transfer.
A vessel for a composition containing magnetic particles and a rod for collecting or transferring particles to be used together with the vessel have now been invented.
The vessel is preferably symmetrical, such as circular, in cross-section, and the cross-sectional shape of the rod matches the cross-sectional shape of the vessel. The rod is equipped with a magnet, by means of which particles are collected on the rod surface, preferably at its tip portion. The magnet is preferably such a magnet whose action can be eliminated, so that the particles can be released from the rod surface.
The magnet is preferably a permanent magnet, and most preferably a magnet that can be shifted between a collecting position and a releasing position within the rod. The direction of movement is especially the longitudinal direction of the rod. The collecting position of the magnet is preferably its lower position, from where it can be raised to the releasing position. The magnet preferably has a length that is significantly greater than its width. This allows particles to be efficiently collected on the rod tip. For the same reason, the upper end of the magnet preferably remains continuously above the composition surface while the particles are collected (cf. WO 96/12958). To this end, the length of the magnet can be clearly greater than the height of the liquid column to be treated. In the magnet, the ratio of its length to its width is preferably at least about 5:1 and most preferably at least about 10:1. The rod preferably slopes downwards at every point, ending in a sharp tip. In this manner, the rod, when lifted from the liquid, tends to retain as small an amount of liquid as possible (cf. WO 94/18565 and WO 94/18564). The use of a long magnet allows more efficient operation in small volumes. A long magnet is useful also in large volumes, because it enlarges the dynamic area at both ends. The tip preferably also has a tapered concave tip portion, so that particles are efficiently released from the rod tip into a small vessel of the same order of magnitude as the tip portion of the rod (cf. WO 96/12959). For the magnetic field to be aligned with this tip portion as efficiently as possible, the height of the tip portion is relatively small compared to the diameter of the magnet. Typically, the ratio of the height of the tip portion to the diameter of the magnet is 1:1-1:2, such as about 1:1.5. The flat shape of the tip also has the purpose of achieving a reliable operating volume that is as small as possible.
Particles can be collected with a specific apparatus, in which the vessels can be placed and which comprises means for moving the rods (cf. WO 94/18565).
Usually a plurality of vessels has been joined to form a plate, especially a matrix-shaped plate, which comprises several rows of vessels in succession. A commonly used plate is the one called micro-titration plate, comprising 8*12 vessels, or wells, at 9 mm intervals. A plate with corresponding outer dimensions and 16*24 wells is also used. The apparatus intended for the treatment of the plates may comprise a row of rods equal to the number of wells in one row (cf. WO 94/18565 and Thermo Labsystems Kingfisher(trademark) Magnetic Particle Processor). The apparatus also allows the operation of several, such as two, plates simultaneously, especially in parallel.
According to a first feature of the invention, the interstice between the vessel and the rod is large enough to prevent the formation of a liquid ring or a ring part rising along with the rod and thereby removing liquid from the vessel. In other words, the liquid flows down between the rod and the vessel under the force of gravity. Ascending water could flow over the edge of the vessel e.g. into an adjacent vessel or in the tip of a removed tip to the subsequent process step. A suitable interstice between the rod and the vessel can be at least about 1 mm, such as at least about 1.5 mm, and preferably at least about 2 mm. In common cases, an interstice above 3 mm does not gain any additional benefit in this respect.
According to a second feature of the invention, the vessel bottom comprises a positioning recess, into which the rod tip is fitted with the rod at a sufficient distance from the inner wall of the vessel at every point. The positioning recess serves to compensate for positioning errors caused by the horizontal movement of the rod and the vessel. The recess centre preferably comprises a horizontal area, whose width corresponds to the positioning margin. The recess shape is preferably identical to that of the tip portion of the rod, so that the interstice between the recess and the tip portion becomes as small as possible. The interstice is preferably 0.05-0.3 mm at the most, such as 0.1-0.2 mm. When a symmetrical vessel and rod are used, the recess will be located at the centre of the vessel. The horizontal area typically has a width in the range 0.5-2 mm, such as about 1 mm. However, the rod is most preferably such that it is usable also in conventional vessels with a flat or concave bottom of various sizes.
According to a third feature of the invention, the apparatus in which the vessel is treated by means of the rod, comprises a vertically elastic means, which yields as the rod is lowered against the vessel bottom. This allows compensation for the positioning error caused by the positioning tolerances of the vertical movement and by the manufacturing tolerances of the rod and the vessel. The elastic means, such as spring means, may be provided in a base included in the apparatus for the vessel, in the rod or its actuating gear, or in a plate comprising several vessels.
The invention is suitable for use especially during operations in very small volumes. The lower limit may be e.g. 10 xcexcl or even 5 xcexcl. The invention is suitable for use also when particles are collected from a relatively great volume, e.g. a bacterial culture, and are transferred into a volume that may be up to many times smaller.